0:15Skip to 0 minutes and 15 secondsZika is a vector borne disease similar to other arboviruses such as chikungunya, Zika is transmitted to humans via a mosquito. When a female mosquito takes a bloodmeal from a person infected with the Zika virus, the virus can survive and multiply within the mosquito. It is then passed on to another person through the mosquito's saliva when she takes her next bloodmeal. Unlike other vector-borne diseases such as malaria lymphatic filariasis and dengue, we know very little about Zika transmission. The Zika virus has been found in several different species of mosquitoes, but it's important to determine which species of the principal vectors in an area of transmission so that we can use suitable tools to control them.

1:01Skip to 1 minute and 1 secondSome of the potential vectors bite people outdoors in the day, but some may come indoors and bite people at night. Tools that target these different behaviors discussed later in the course. First we need to know which mosquitoes are involved in transmission and this process is known as vector incrimination. There are four main criteria that need to be satisfied to fully incriminate a suspected species as a vector of Zika. This can be done indirectly by molecular analysis of bloodfed mosquitoes that have been collected from the field and you will be shown different methods of collecting adult mosquitoes later in the course. Second, you need to know that the distribution of the suspected vector coincides with the distribution of Zika.

1:50Skip to 1 minute and 50 secondsThe third step involves detecting the Zika virus using molecular screening techniques in mosquitoes that have been collected from the field site. And you need to check whether the strain of Zika found in the mosquitoes is the same as that found in human cases. The fourth step is to demonstrate that the mosquitoes a competent vector. This informs laboratory studies in a special containment facility. Mosquito females are allowed to feed on a bloodmeal containing the Zika virus used in an artificial membrane feeding system. Over time, the mosquitoes are tested to see whether the virus survives and multiplies inside them. After several days the saliva of the mosquitoes is tested to see whether it has the Zika virus.

2:38Skip to 2 minutes and 38 secondsIn some studies, experimentally-infected mosquitoes are then fed on a test animal such as a mouse to see whether they can pass on the virus. So far no species have been fully incriminated as vectors of Zika, but the following slides present evidence to suggest that certain species may be vectors.

3:00Skip to 3 minutes and 0 secondsThe first isolation Zika virus was from Aedes africanus mosquitoes collected in Zika forest, Uganda, way back in 1948. The virus was isolated from rhesus monkeys found in the forest at the same time. A study published in 1969 isolated Zika virus from a different species of Aedes, Aedes aegypti, from mosquito collections in a variety of urban and rural habitats in Malaysia. Since then the virus has been isolated from seven other Aedes species collected in Uganda, Nigeria, Gabon and Senegal. However, Aedes mosquitoes may not be the only genus capable of transmitting Zika because the virus has also been isolated from Mansonia uniformis, Anopholes coustani, and Culex perfuscus.

4:00Skip to 4 minutes and 0 secondscompetence only use two species: Aedes aegypti and Aedes albopictus. One reason for this is that both species are easy to rear in the laboratory. In 1956, Aedes aegypti mosquitoes were the first mosquito species to be infected with Zika virus artificially in a laboratory. Females were fed on virus infected blood, treated with heparin to prevent it clotting and through mouse membrane. The virus was detectable 20 days after they received the bloodmeal and the mosquitoes passed on the virus to mice and rhesus monkeys in the laboratory. Later that year, Aedes aegypti mosquitoes experimentally infected with the virus were shown to infect a human volunteer.

4:57Skip to 4 minutes and 57 secondsIn Singapore a study was performed to determine whether local Aedes albopictus populations could transmit Zika virus if it was transported to that country. And the species has shown to be a competent vector. Virus was present in saliva seven days after the mosquitoes were given an uninfected bloodmeal. More recently, a study comparing the competence of both species using populations originally from Guadeloupe and French Guyana concluded that although both species were competent they had low transmission rates. Zika virus has been isolated from natural mosquito populations of several species mostly Aedes genus, but several of the other genera can also be infected. Published laboratory studies have only been performed using Aedes aegypti or Aedes albopictus.

5:51Skip to 5 minutes and 51 secondsBoth species are competent but neither has been fully incriminated. It's too early to say whether one mosquito species is a principal vector in a particular transmission setting. And we should not overlook the likelihood of other mosquito species or indeed of the biting diptera being involved in transmission. Further work is required to fully incriminate the vector, and they should involve making contemporary mosquito collections, screening them for arboviruses, and studying Zika infections and the human populations, taking into account the lag between exposure to virus and manifestation of symptoms particularly with regards to microcephaly. We also cannot ignore other possible routes of transmission.

6:46Skip to 6 minutes and 46 secondshumans, or zoonotic: that means it involves a non human reservoir host in addition to vectors in humans. People can pass the virus on to another person without vector involvement.

7:08Skip to 7 minutes and 8 secondsIn addition two other routes have been proposed: the first involves sexual transmission. The have been at least three case reports of men returning home from Zika endemic areas (Senegal, French Polynesia, and Brazil) to non endemic areas with high infectious Zika viral loads in their semen. It appears that in one case Zika was sexually transmitted to his wife who was living in a non endemic area. The second involves breastfeeding from a case report showing the presence of Zika virus in the breast milk in a woman who gave birth to a healthy baby while febrile and immediately breastfed. The relative importance of direct human-to-human transmission versus vector transmission is unknown but needs to be addressed to direct control efforts.

How is Zika spread?

Zika is a vector-borne disease that is similar to other viruses, such as dengue and chikungunya, and like these is also transmitted to humans via a mosquito. However, unlike other vector-borne diseases such as malaria, we know very little about Zika transmission, including its principal vector.

In the video Dr Mary Cameron outlines the known history of Zika and the associations it has had with different mosquito species. She also discusses the type of evidence that scientists need to gather in order to fully incriminate any one species as a vector. She then goes on to consider other modes of transmission that may be significant but do not involve insect vectors.

Why do you think it is important to determine the primary vectors of Zika?